This article describes how to use automatic Transmission Control Protocol/Internet Protocol (TCP/IP) addressing without a Dynamic Host Configuration Protocol (DHCP) server being present on the network. The operating system versions listed in the "Applies to" section of this articlehave a feature called Automatic Private IP Addressing (APIPA). With thisfeature, a Windows computer can assign itself an Internet Protocol (IP)address in the event that a DHCP server is not available or does not exist onthe network. This feature makes configuring and supporting a small Local AreaNetwork (LAN) running TCP/IP less difficult.
Serious problems might occur if you modify the registry incorrectly by using Registry Editor or by using another method. These problems might require that you reinstall your operating system. Microsoft cannot guarantee that these problems can be solved. Modify the registry at your own risk.
A Windows-based computer that is configured to use DHCP can automaticallyassign itself an Internet Protocol (IP) address if a DHCP server is notavailable. For example, this could occur on a network without a DHCP server, oron a network if a DHCP server is temporarily down for maintenance.
The Internet Assigned Numbers Authority (IANA) has reserved 169.254.0.0-169.254.255.255 for Automatic Private IP Addressing. As a result, APIPA provides an address that is guaranteed not to conflict with routable addresses.
After the network adapter has been assigned an IP address, the computer can use TCP/IP to communicate with any other computer that is connected to the same LAN and that is also configured for APIPA or has the IP address manually set to the 169.254.x.y (where x.y is the client’s unique identifier) address range with a subnet mask of255.255.0.0. Note that the computer cannot communicate with computers on othersubnets, or with computers that do not use automatic private IP addressing.Automatic private IP addressing is enabled by default.
You may want to disableit in any of the following cases:
- Your network uses routers.
- Your network is connected to the Internet without a NAT or proxy server.
Unless you have disabled DHCP-related messages, DHCP messages provide you withnotification when you change between DHCP addressing and automatic private IPaddressing. If DHCP messaging is accidentally disabled, you can turn the DHCPmessages back on by changing the value of the PopupFlag value in the followingregistry key from 00 to 01:
Note that you must restart your computer for the change to take effect. You canalso determine whether your computer is using APIPA by using the Winipcfg tool in Windows Millennium Edition, Windows 98, or Windows 98 Second Edition:
Click Start, click Run, type "winipcfg" (without the quotation marks), and then click OK. Click More Info. If the IP Autoconfiguration Address box contains an IP address within the 169.254.x.x range, Automatic Private IP Addressing is enabled. If the IP Address box exists, automatic private IP addressing is not currently enabled.
For Windows 2000, Windows XP, or Windows Server 2003, you can determine whether your computer is using APIPA by using the IPconfig command at a command prompt:
Click Start, click Run, type "cmd" (without the quotation marks), and then click OK to open a MS-DOS command line window. Type "ipconfig /all" (without the quotation marks), and then hit the ENTER key. If the 'Autoconfiguration Enabled' line says "Yes", and the 'Autoconfiguration IP Address' is 169.254.x.y (where x.y is the client's unique identifier), then the computer is using APIPA. If the 'Autoconfiguration Enabled' line says "No", then the computer is not currently using APIPA.
You can disable automatic private IP addressing by using either of the followingmethods.
You can configure the TCP/IP information manually, which disables DHCP altogether. You can disable automatic private IP addressing (but not DHCP) by editing the registry. You can do so by adding the "IPAutoconfigurationEnabled" DWORD registry entry with a value of 0x0 to the following registry key for Windows Millennium Edition, Windows98, or Windows 98 Second Edition:
For Windows 2000, Windows XP, and Windows Server 2003, APIPA can be disabled by adding the "IPAutoconfigurationEnabled" DWORD registry entry with a value of 0x0 to the following registry key:
The Adapter GUID
subkey is a globally unique identifier (GUID) for the computer's LAN adapter.
Specifying a value of 1 for IPAutoconfigurationEnabled DWORD entry will enable APIPA, which is the default state when this value is omitted from the registry.
Examples of Where APIPA May Be Useful
Example 1: No Previous IP Address and no DHCP Server
When your Windows-based computer (configured for DHCP) is initializing, it broadcasts three or more "discover" messages. If a DHCP server does not respondafter several discover messages are broadcast, the Windows computer assigns itself a Class B (APIPA) address. Then the Windows computer will display an error message to the user of the computer (providing it has never been assigned an IP address from a DHCP server in the past). The Windows computer will then send out a Discover message every three minutes in an attempt to establish communications with a DHCP server.
Example 2: Previous IP Address and no DHCP Server
The computer checks for the DHCP server and if none are found, an attempt is madeto contact the default gateway. If the default gateway replies, then the Windowscomputer retains the previously-leased IP address. However, if the computer does not receive a response from the default gateway or if none are assigned,then it uses the automatic private IP addressing feature to assign itself an IPaddress. An error message is presented to the user and discover messages aretransmitted every 3 minutes. Once a DHCP server comes on line, a message isgenerated stating communications have been re-established with a DHCP Server.
Example 3: Lease Expires and no DHCP Server
The Windows-based computer tries to re-establish the lease of the IPaddress. If the Windows computer does not find a DCHP server, it assignsitself an IP address after generating an error message. The computer then broadcasts four discover messages, and after every 5 minutes it repeats the whole procedure until a DHCP server comes on line.A message is then generated stating that communications have been re-establishedwith the DHCP Server.